Bump tox from 3.19.0 to 3.20.0 (#193)

Bumps [tox](https://github.com/tox-dev/tox) from 3.19.0 to 3.20.0.
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a href="https://github.com/tox-dev/tox/blob/master/docs/changelog.rst">tox's changelog</a>.</em></p>
<blockquote>
<h2>v3.20.0 (2020-09-01)</h2>
<p>Bugfixes
^^^^^^^^</p>
<ul>
<li>Allow hyphens and empty factors in generative section name. - by :user:<code>tyagdit</code>
<code>[#1636](tox-dev/tox#1636) &lt;https://github.com/tox-dev/tox/issues/1636&gt;</code>_</li>
<li>Support for PEP517 in-tree build backend-path key in <code>get-build-requires</code>. - by :user:<code>nizox</code>
<code>[#1654](tox-dev/tox#1654) &lt;https://github.com/tox-dev/tox/issues/1654&gt;</code>_</li>
<li>Allow escaping curly braces in setenv. - by :user:<code>mkenigs</code>
<code>[#1656](tox-dev/tox#1656) &lt;https://github.com/tox-dev/tox/issues/1656&gt;</code>_</li>
</ul>
<p>Features
^^^^^^^^</p>
<ul>
<li>Support for comments within <code>setenv</code> and environment files via the <code>files|</code> prefix. - by :user:<code>gaborbernat</code>
<code>[#1667](tox-dev/tox#1667) &lt;https://github.com/tox-dev/tox/issues/1667&gt;</code>_</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/tox-dev/tox/commit/5c94919b79105eabb0507f6d5e55b60daee11ccc"><code>5c94919</code></a> release 3.20.0</li>
<li><a href="https://github.com/tox-dev/tox/commit/8a25fa6bf3e625c0b47659e1652ab55a79d4d1e8"><code>8a25fa6</code></a> Support environment files and comments in setenv (<a href="https://github-redirect.dependabot.com/tox-dev/tox/issues/1668">#1668</a>)</li>
<li><a href="https://github.com/tox-dev/tox/commit/a7903508fa07068b327e15cfdbf8ea330ab78765"><code>a790350</code></a> New formatters</li>
<li><a href="https://github.com/tox-dev/tox/commit/d4e6e77ea77cf8b336f38313bef40c80b6771139"><code>d4e6e77</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/tox-dev/tox/issues/1656">#1656</a> from mkenigs/escape-braces-in-setenv</li>
<li><a href="https://github.com/tox-dev/tox/commit/8e39dfbb836b6e2e6b2bd637f7b853935e82de3f"><code>8e39dfb</code></a> Spelling fixes (<a href="https://github-redirect.dependabot.com/tox-dev/tox/issues/1665">#1665</a>)</li>
<li><a href="https://github.com/tox-dev/tox/commit/00f6496b756f938db4ddcf040a5192a9e40b16a4"><code>00f6496</code></a> Document allowlist_externals versionadded (<a href="https://github-redirect.dependabot.com/tox-dev/tox/issues/1663">#1663</a>)</li>
<li><a href="https://github.com/tox-dev/tox/commit/3c38762fc6acfd6ca93ddc8607133d3a39e6d25c"><code>3c38762</code></a> Allow escaping curly braces in setenv</li>
<li><a href="https://github.com/tox-dev/tox/commit/3453018dab9a0208f64013c2dd357d1be348ffdf"><code>3453018</code></a> Make setup.py test admonition sterner as it is deprecated (<a href="https://github-redirect.dependabot.com/tox-dev/tox/issues/1660">#1660</a>)</li>
<li><a href="https://github.com/tox-dev/tox/commit/4b7f682db872902b542ccaf26d9619c141dfc8ab"><code>4b7f682</code></a> remove broken link to removed pytest-runner documentation (<a href="https://github-redirect.dependabot.com/tox-dev/tox/issues/1659">#1659</a>)</li>
<li><a href="https://github.com/tox-dev/tox/commit/f292f5236a41f7c02ba60dbc14d8e5941be7ea0e"><code>f292f52</code></a> Fix support of the PEP517 backend-path key (<a href="https://github-redirect.dependabot.com/tox-dev/tox/issues/1655">#1655</a>)</li>
<li>Additional commits viewable in <a href="https://github.com/tox-dev/tox/compare/3.19.0...3.20.0">compare view</a></li>
</ul>
</details>
<br />


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</details>

setup.py

@@ -24,6 +24,6 @@
         "rich==5.2.1",
     ],
     extras_require={
-        "dev": ["black==20.8b1", "flake8==3.8.3", "moto==1.3.14", "pre-commit==2.7.1", "pytest==6.0.1", "tox==3.19.0"]
+        "dev": ["black==20.8b1", "flake8==3.8.3", "moto==1.3.14", "pre-commit==2.7.1", "pytest==6.0.1", "tox==3.20.0"]
     },
 )